Growth of the gastrointestinal mucosa is markedly influenced by nutritional status and enteral nutrient availability. This is evidenced by the disproportionate loss of gut mucosal mass relative to body weight during starvation and other states of malnutrition (1-2). Fasting or severe protein-calorie restriction result in mucosal cell atrophy, decreased digestive enzyme activity and absorptive capacity, and impaired intestinal barrier function (3-4). Malnutrition is also associated with reduced antioxidant capacity in the intestinal mucosa (5). Enteral refeeding after a period of malnutrition rapidly regenerates intestinal cellularity and mucosal mass (3-5).
The tripeptide glutathione (L-glutamyl-L-cysteinyl-glycine, GSH) is the most abundant low molecular weight thiol in mammalian cells and plays a key role in the detoxification of cellular free radicals, chemical toxins, and carcinogens (16). GSH deactivates potentially harmful oxidants by serving as a hydrogen donor to reduce reactive molecules with concomitant conversion to its oxidized disulfide form, GSSG (6). GSH is synthesized endogenously in mucosal cells utilizing specific amino acid substrates, can be derived exogenously from dietary sources, or may enter the gut lumen via bile and by direct secretion from mucosal cells (7-8). GSH present in the gut lumen and within enterocytes appears to be required for normal intestinal function, in part, by protecting intestinal epithelial cells from damage by dietary electrophiles and fatty acid hydroperoxides (9-11). GSH also appears to play a role in maintaining the proper sulfhydryl/disulfide balance of gut luminal proteins, potentially modulating activity of thiol-containing enzymes on the brush border (12-13).
Previous studies demonstrate that malnutrition reduces tissue GSH content (5, 15-16). In animal models, fasting or an insufficient dietary supply of amino acids that may serve as GSH substrates (e.g., glutamine and cysteine) depletes GSH levels in both small intestine and colon (5, 16, 17). Therefore, malnutrition-associated depletion of cellular GSH in gut epithelial cells may increase their susceptibility to oxidative injury and exacerbate the degeneration of the intestinal mucosa (17). Also, there is evidence to suggest that GSH is involved in regulation of cell growth (18).
In studies using a variety of cultured mammalian cells, a more reduced state of the extracellular GSH pool was associated with increased cell proliferation, while a more oxidized GSH pool was associated with slower cell growth (18). Intracellular and extracellular antioxidant status also appears to influence cell proliferation mediated by specific growth factor peptides, including platelet-derived growth factor (PDGF) and epidermal growth factor (EGF) (19-22). It is therefore possible that the reducing environment regulated by GSH in gut mucosa may be important not only for detoxification reactions allowing normal tissue growth and function, but also for regulating cell proliferation in response to nutrients and growth factors.
Keratinocyte growth factor (KGF), a member of the fibroblast growth factor (FGF) family, is a mesenchymally-derived peptide which appears to be an important endogenous mediator of epithelial growth, regeneration and repair (23). It is one example of a gut-trophic growth factor. Exogenous administration of recombinant human KGF in cell culture systems or in in vivo animal models stimulates proliferation and differentiation of specific epithelial cell types, including hepatocytes and enterocytes, and also appears to have cytoprotective functions (24). In healthy rats fed ad libitum diets, administration of KGF induced epithelial cell proliferation in the stomach, duodenum, colon, liver and pancreas (25).
Administration of KGF enhances small intestinal and colonic mucosal growth during enteral refeeding after a 3-day period of fasting (26). The mechanisms by which KGF acts as a potent gut mitogen during enteral nutrition are unclear. The current study is designed to investigate mucosal GSH status associated with gut growth stimulated by enteral nutrition and by GTGF(s) in a fasting/refeeding rat model. The major aims of this study were: 1) to determine whether different levels of enteral refeeding changes small intestinal and colonic mucosal levels of GSH and GSSG and the GSH redox potential; and 2) to assess the effects of the gut-trophic growth factor(s) on mucosal GSH antioxidant capacity in models of altered enteral nutrition. A further aim was to determine whether changes in mucosal GSH status are associated with changes in indices of mucosal growth.
There is a strong need for methods for treating patients and animals suffering from malnutrition, starvation and/or malabsorption, especially during refeeding after a period of insufficient nutrition, and there is also a longfelt need in the art for methods of treatment which result in an improvement in local and/or systemic improvement in the oxidation state, particularly as measured by the glutathione/reduced glutathione ratio, due to age, disease, catabolic stress, sequelae to certain medical treatment regiments, trauma, inflammation, among other conditions. The present invention meets that need.
The present invention provides a method for the improvement of systemic oxidation-reduction state, as measured in the study described hereinbelow by the GSH and GSSG concentrations and the GSH reducing potential, as well as an improvement of the oxidation-reduction state of the gastrointestinal epithelial cells, for example, during refeeding after fasting, malnutrition or other stress. The present method includes the step of administering an amount of at least one gut-trophic growth factor by a suitable means. Gut-trophic growth factors include keratinocyte growth factor, hepatocyte growth factor, insulin-like growth factor I and glucagon, glicentin and a glucagon-like peptide. GTGFs further include fibroblast growth factors: acidic fibroblast growth factor, basic fibroblast growth factor, fibroblast growth factor-4, fibroblast growth factor-5, fibroblast growth factor-6, fibroblast growth factor-9, fibroblast growth factor-10, fibroblast growth factor-10 and hst/K fibroblast growth factor. Desirably, the gut-trophic growth factor is substantially identical, preferably identical, corresponds in amino acid sequence to that of the animal or human to whom it is administered. Suitable routes of administration include, but are not limited to, enterally, intraperitoneally or intravenously.
The present invention further provides methods for the treatment of medical (or veterinary) conditions so as to improve the general, tissue and systemic antioxidant oxidation status, the conditions including, but not limited to, aging, general malnutrition associated with aging, disease, malabsorption disorders or psychological dysfunction, inflammatory bowel disease, chemotherapy, radiation exposure or therapy, corticosteroid therapy, toxin ingestion, alcoholism and inflammation associated with burns, sepsis, infection or trauma. Administration (parenterally or enterally) of an effective amount of a composition comprising at least one gut-trophic growth factor improves antioxidant capacity and antioxidant status as measured by the tissue or serum GSH and GSSG levels. The oxidation-reduction status can be measured as specifically exemplified herein, by GSH and GSSG concentrations and the calculated GSH redox state in plasma or in a tissue sample of a human patient or animal can be calculated.